1. Field of the Invention
This invention relates to a ferroelectric material, more particularly to a ferroelectric material having a lead barium zirconate-based modified structure so as to improve fatigue resistance thereof. This invention also relates to a ferroelectric memory device made from the ferroelectric material.
2. Description of the Related Art
Nowadays, in view of rapid development in semiconductor processing techniques, there is a surge of interest in searching for ferroelectric materials to meet the requirements for reducing volume and size of a memory device. In recent development of memories, attention has been drawn to the manufacture of non-volatile semiconductor memories by using ferroelectric materials that exhibit a polarization hysteresis having two stable electrical polarization states.
In application of the ferroelectric materials, such as lead zirconate titanate, to a ferroelectric memory device, since lead zirconate titanate (Pb(Zrx, Ti1−x)O3, PZT) is characterized by a relatively high remanent polarization (Pr) and a high Curie temperature (Tc) with respect to the operating temperature, PZT is useful for manufacturing of the ferroelectric memory device having a Pt/PZT/Pt capacitor. However, although such ferroelectric memory device has high remanent polarization, the switched polarization (ΔP) will decrease with an increase in switching cycles after repeated switching at a predetermined voltage. As such, the applied coercive field (Ec) of the ferroelectric memory device is required to be increased accordingly, which, in turn, results in poor fatigue resistance.
In detail, since oxygen vacancies accumulated at the interface between the PZT film and the metal electrode layers during switching cycles, movement of domains included in the ferroelectric memory device tends to be adversely affected, which can result in deterioration of the fatigue resistance. Formation of such oxygen vacancies is related to reduction of Ti4+ to Ti3+ in PZT during manufacture of the ferroelectric memory device. Conductive metal oxides, such as (La, Sr) CoO3 (LSCO), SrRuO3 (SRO), and YBa2Cu3O7−x (YBCO), that have a perovskite structure similar to that of PZT, can be used for manufacturing metal electrode layers, so as to improve the fatigue resistance of the ferroelectric memory device.
For example, U.S. Pat. Nos. 5,519,235, 5,248,564 and 5,838,035 describe a ferroelectric memory device including a LSCO/PZT/LSCO capacitor. U.S. Pat. Nos. 6,194,228 and 5,155,658 describe a ferroelectric memory device including a SRO/PZT/SRO capacitor and a YBCO/PZT/YBCO capacitor, respectively. The entire contents of these prior art Patents are incorporated herein by reference.
However, even though the fatigue resistance of the ferroelectric memory device mentioned in these prior art Patents can be improved by using the conductive metal oxides as the electrode layers, the manufacture of such ferroelectric memory device requires a relatively high sputtering or depositing temperature, e.g., about 600° C. to 810° C., and complicated processing steps.
Therefore, there is still a need in the art to provide a ferroelectric material that is capable of forming a ferroelectric memory device with good fatigue resistance and high remanent polarization.